Shaped nanocrystal particles and methods for making the same

ABSTRACT

Shaped nanocrystal particles and methods for making shaped nanocrystal particles are disclosed. One embodiment includes a method for forming a branched, nanocrystal particle. It includes (a) forming a core having a first crystal structure in a solution, (b) forming a first arm extending from the core having a second crystal structure in the solution, and (c) forming a second arm extending from the core having the second crystal structure in the solution.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11,869,585, filed on Oct. 9, 2007 now U.S. Pat, No. 8,062,421, which iscontinuation of U.S. patent application Ser. No. 10/980,472, filed onNov. 2, 2004 now U.S. Pat. No. 7,311,774, which is a divisional of U.S.patent application Ser. No. 10/301,510, filed on Nov. 20, 2002 now U.S.Pat, No. 6,855,202, which is a non-provisional application based on U.S.Provisional Patent Application No. 60/335,435, filed on Nov. 30, 2001.These applications are herein incorporated by reference in theirentirety for all purposes.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH OR DEVELOPMENT

The invention described and claimed herein was made in part utilizingfunds supplied by the United States Department of Energy under contractNO. DE-AC03-76SF000-98 between the United States Department of Energyand The Regents of the University of California. The government hascertain rights to the invention.

BACKGROUND OF THE INVENTION

The ability to systematically manipulate the shapes of inorganicnanocrystal particles remains a goal of modern materials chemistry. Theshape and size of inorganic nanocrystal particles control their widelyvarying electrical and optical properties. One means of achieving shapecontrol is through the use of a static template to enhance the growthrate of one crystallographic face over another. For example,two-dimensional films are obtained when there is favorable epitaxy on asubstrate (Cho, J. Cryst., Growth, 202:1-7 (1999)). Pyramidal “dots” areobtained if there is strain between the growing crystallite and theepitaxial substrate, as in the growth of InAs on GaAs (Leon et al.,Science, 267:1966-1968 (1995)) and Ge on Si (Liu et al., Phys. Rev.Lett., 84:1958-1961 (2000)).

Anisotropic inorganic nanocrystal particles have also been grown inliquid media. The vapor-liquid-solid growth mechanism in which a solidrod grows out of a supersaturated droplet has been used to createone-dimensional materials (Hu et al., Accounts of Chemical Research,32:435-445 (1999)), and has been applied to the growth of (insoluble)nanorods in a liquid medium (Trentler et al., Science, 270:1791-1794(1995); Holmes et al., Science, 287:1471-1473 (2000)).

While anisotropic nanocrystal particles are useful, it would bedesirable if nanocrystal particles with other shapes could be formed. Aswill be explained in further detail below, complex shaped nanocrystalparticles such as tetrapods have a number of features that make themmore desirable than nanocrystal rods or spheres for some applications.Other advantages of complex shaped nanocrystal particles are describedbelow.

SUMMARY OF THE INVENTION

Embodiments of the invention are directed to shaped nanocrystalparticles and processes for making shaped nanocrystal particles. Theshaped nanocrystal particles can be branched (e.g., in the form oftetrapods), or can be in the form of teardrops or arrows.

One embodiment of the invention is directed to a process of forming ananocrystal particle, the process comprising: (a) providing a corehaving a first crystal structure in a solution; and (b) forming an armextending from the core having a second crystal structure in thesolution.

Another embodiment of the invention is directed to a process for formingsemiconductor nanocrystal particles comprising: introducingsemiconductor nanocrystal particle precursors into a heated mixture ofsurfactants capable of promoting the growth of tetrapod shapedsemiconductor nanocrystal particles; and forming tetrapod shapedsemiconductor nanocrystal particles.

Another embodiment of the invention is directed to a nanocrystalparticle comprising: a core having a first crystal structure; and atleast an arm extending from the core, and having a second crystalstructure.

Another embodiment of the invention is directed to a branchednanocrystal particle comprising: a core; at least a first arm extendingfrom the core; and at least a second arm extending from the core,wherein the second arm forms a branch with respect to the first arm, andwherein the core, first arm, and the second arm comprise a Group II-VIor a Group III-V semiconductor.

Another embodiment of the invention is directed to a tetrapod shapednanocrystal particle comprising: a core having a first crystalstructure; a first arm extending from the core; a second arm extendingfrom the core; a third arm extending from the core; and a fourth armextending from the core, wherein the first, second, third, and fourtharms have a second crystal structure, wherein the first crystalstructure is different than the second crystal structure.

Another embodiment of the invention is directed to a nanocrystalparticle in the form a teardrop or an arrow.

Another embodiment of the invention is directed to a process for formingshaped nanocrystal particles comprising: (a) mixing semiconductorprecursors and a mixture of surfactants to form a solution; and (b)forming nanocrystal particles in the solution, wherein the nanocrystalparticles are in the form of teardrops or arrows.

These and other embodiments of the invention are described in furtherdetail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a proposed model of a CdTe tetrapod.

FIG. 2( a) shows ensemble optical absorption spectra for a series oftetrapods having comparable arm lengths, but different diameters.

FIG. 2( b) shows ensemble optical absorption spectra for a series oftetrapods having comparable arm diameters, but different lengths.

FIG. 3( a) shows a schematic illustration of a tetrapod according to anembodiment of the invention.

FIG. 3( b) shows a schematic illustration of a branched tetrapodaccording to an embodiment of the invention.

FIG. 3( c) shows a schematic illustration of an inorganic dendrimeraccording to an embodiment of the invention.

FIGS. 4( a)-4(c) show schematic 3-dimensional illustrations of how abranched tetrapod is formed.

FIG. 5 shows a transmission electron micrograph (TEM) of nanocrystalparticles made using 60% HPA in TOPO. Arrow-shaped nanocrystal particlesare shown.

FIGS. 6( a)-6(e) show transmission electron micrographs (TEMs) ofnanocrystal particles produced using 60 mole % HPA (FIG. 6( a)). Highresolution TEM (HRTEM) images show the stages of growth from (FIG. 6(b)) pencil to (FIG. 6( c)) narrow arrow to (FIG. 6( d)) pine-tree shapednanocrystal particles. In FIG. 6( e), a pine-tree shaped nanocrystal isalso shown looking down the [001] direction (or long axis). HRTEMcharacterization shows that each shape of nanocrystal is predominatelywurtzite, and that the angled facets of the arrows are the (101) faces.

FIG. 7 shows a 2-dimensional representation showing the relationshipbetween the wurtzite and zinc blende structures. Wurtzite has ABABstacking while zinc blende has ABCABC stacking. The (111) face of zincblende and the (00 1) face of wurtzite both have planes alternatelycomposed of Cd and Se atoms. The two structures are related by astacking fault.

FIGS. 8( a)-8(c) show transmission electron micrographs (TEMs) of (FIG.8( a)) typical teardrop shaped nanocrystal particles. A high resolutionTEM (HRTEM) image (FIG. 8( b)) shows the wurtzite structure of particlesthat are teardrop shaped. FIG. 8( c) shows a HRTEM image of ananocrystal, which after an additional injection shows growth on boththe (001) and (00 1) faces. The center of this particle is zinc blendein structure.

FIGS. 9( a)-9(b) show high resolution transmission electron microscope(HRTEM) images of tetrapod nanocrystal particles. FIG. 9( a) shows atypical tetrapod shaped CdSe nanocrystal particle, looking down the[001] direction of one arm. Lattice spacings confirm that all four armsare of the wurtzite structure. FIG. 9( b) shows a tetrapod that hasbranches growing out of each arm. There are zinc blende layers near theends of the original arms and the branches are wurtzite with somestacking faults.

FIG. 10 shows an atomic model of CdSe in the wurtzite structuredemonstrating the differences between the (001) and (00 1) faces. On the(001) face, Cd atoms have only one dangling bond, while on the (00 1)face, Cd atoms have three dangling bonds that need to be passivated.

FIG. 11 shows a 2-dimensional representation showing the structure of atetrapod. The nuclei is the zinc blende structure, with wurtzite armsgrowing out of each of the four (111) equivalent faces. Three are shown,with the fourth coming out of the page towards the reader.

FIG. 12 shows transmission electron micrographs (TEMs) of CdTe tetrapodsgrown under various reaction conditions.

FIG. 13 shows TEMs of CdTe tetrapods extracted from the same synthesisat 1 and at 5 minutes, respectively, for two syntheses carried out atthe same Cd/Te ratio (5:1), but at two different Cd/ODPA ratios (1:2 and1:5).

FIG. 14 shows a typical powder X-ray diffraction (XRD) of a CdTetetrapod sample (non-vertical line with peaks). The bulk XRD pattern ofCdTe wurtzite is also shown (vertical lines).

DETAILED DESCRIPTION

In embodiments of the invention, the shapes of colloidal semiconductornanocrystal particles can be systematically varied using the thermaldecomposition of organometallic precursors in a hot mixture ofsurfactants. The hot mixture of surfactants may comprise, for example,trioctylphosphine oxide (TOPO) and an alkylphosphonic acid. As in thegrowth of spherical CdSe nanocrystal particles in hot trioctylphosphineoxide, the surfactants dynamically adsorb to the growing crystallites,allowing atoms to add and subtract for high crystallinity. This enablesthe growing crystallites to anneal, resulting in good crystallinity,while suppressing particle aggregation.

Controlled nanocrystal particle growth can depend on a number offactors. For example, the growth mode of the nanocrystal particles canbe controlled by adjusting the monomer concentration. At low monomerconcentration, Ostwald ripening occurs, and small nanocrystal particlescan dissolve at the expense of larger ones. Such slow growth conditionsfavor the formation of a spherical particle shape (i.e., the nanocrystalparticles want to form in a way that minimizes surface area). On theother hand, at high monomer concentration, relative differences betweenthe growth rates of different faces can lead to anisotropic shapes.Using control mechanisms such as this, the present inventors hereindemonstrate surprisingly the controlled formation of nanocrystalparticles with tetrapod, teardrop, and arrow shapes.

As used herein, “nanocrystal particles” can refer to crystallineparticles that have at least one dimension less than about 100nanometers. In some embodiments of the invention, the nanocrystalparticles may have two or more dimensions that are less than about 100nanometers. For example, the branched nanocrystal particles according tosome embodiments of the invention can have arms that have aspect ratiosgreater than about 1. In other embodiments, the arms can have aspectratios greater than about 5, and in some cases, greater than about 10,etc. The widths of the arms may be less than about 200, 100, and even 50nanometers in some embodiments. For instance, in an exemplary tetrapodwith a core and four arms, the core can have a diameter from about 3 toabout 4 nanometers, and each arm can have a length of from about 4 toabout 50, 100, 200, 500, and even greater than about 1000 nanometers. Ofcourse, the tetrapods and other nanocrystal particles described hereincan have other suitable dimensions. In embodiments of the invention, thenanocrystal particles may be single crystalline or polycrystalline innature.

The nanocrystal particles according to embodiments of the invention canhave unique optical, electrical, magnetic, catalytic, and mechanicalproperties, and can be used in a number of suitable end applications.They can be used, for example, as fillers in composite materials, ascatalysts, as functional elements in optical devices, as functionalelements in photovoltaic devices (e.g., solar cells), as functionalelements in electrical devices, etc.

I. Branched Nanocrystal Particles

Snowflakes are a familiar example in which a solid is formed with a highdegree of branching. The branching of snow crystals is due to growth farfrom equilibrium, at high supersaturation levels of water. Higher levelsof complexity arise when the growing snow crystals experience regions ofdifferent temperature and partial pressure of water as they fall,changing the relative growth rates of the different crystallographicfacets.

Like snowflakes, embodiments of the invention can exhibit polytypism, orthe existence of two or more crystal structures in different domains ofthe same crystal. Polytypism can be exploited to produce branchedinorganic nanostructures in a controlled way. Frequently, polytypicstructures share a common crystal facet, which is desirable forbranching. In conventional macroscopic inorganic crystal growth, thereare few examples of the controlled formation and growth of polytypicstructures. There are also few examples of modulating growth rates ofdifferent crystal facets of a solid as a function of time. However, newmethods for preparing inorganic nanocrystal particles withwell-controlled sizes and shapes (e.g., spheres, rods, disks, and cubes)provide tools that can be adapted to form unique nanocrystal particles.

The tools can be used during the formation of nanocrystal particles topromote the stability of a certain phase over another and hence theformation of one crystal phase over another. This creates a newopportunity to generate artificial inorganic nanostructures withdeliberately designed branches and interconnections.

Polytypism is generally prevalent in open, tetrahedrally bondedstructures such as those occurring in the Group IV, III-V and II-VIsemiconductors. In those semiconductors, there is only one type ofchemical bond with local tetrahedral geometry. The tetrahedral buildingblocks are arranged in puckered rings, are comprised either of allchairs (cubic or zinc blende case) or are mixtures of chairs and boats(hexagonal or wurtzite case). The cubic and hexagonal structures differonly by their second nearest neighbor. The ±{111} facets of the cubiccrystal of these materials are atomically identical to the ±(0001)facets of the hexagonal structure. These identical facets allow ananocrystal particle to start growing with one type of crystal structure(e.g., a cubic crystal structure) and then transition to form a secondtype of crystal structure (e.g., a hexagonal crystal structure).

While the nanocrystal particles of embodiments of the invention can haveany suitable material, CdTe is a particularly suitable candidatematerial for controlled branching. This is because this material has anappropriate energy difference between zinc blende and wurtzitestructures. The energy difference between zinc blende and wurtzite inthe bulk can be larger than 10 meV per atom for the most covalently orthe most ionic structures (such as Si, GaAs, or ZnO), which rarely showpolytypism. In contrast, moderately ionic structures, such as CdS andZnS, occur almost indiscriminately in both phases due to an energydifference of only a few meV. In CdTe, this value is about 7 meV/atom,offering the possibility to control polytypism more easily. In CdTe, thecubic crystal structure is intrinsically more stable than the hexagonalcrystal structure at the temperatures in which they can be grown inorganic solution. Nucleation occurs in the cubic crystal phase, eventhough processing conditions may favor the growth of a hexagonal crystalstructure. In embodiments of the invention, the energy differencebetween different crystal structures in a particular material ispreferably less than 20 meV/atom, less than 15 meV/atom, or less than 10meV/atom so that polytypic structures can be created.

For CdTe and other materials, if the growth of the nanocrystal particletakes place at high temperatures (e.g., greater than about 290° C. forCdTe) and/or at a suitably high monomer concentration and/or in thepresence of a surfactant that promotes it, then the growth of ahexagonal wurtzite structure can be favored over a cubic zinc blendestructure. (It is understood that the processing temperatures may varydepending upon the particular material produced.) These surfactantmolecules are known to selectively stabilize the facets perpendicular tothe c-axis of hexagonal nanocrystal particles. This stabilizationconsiderably reduces the growth rate of these facets, which have noequivalent in the cubic structure. Thus, in the presence of phosphonicacid, nanocrystal particles such as CdTe nanocrystal particles nucleatezinc blende and grow in the wurtzite phase. Here, the present inventorsdemonstrate the reproducible synthesis, in high yield, of tetrapodshaped nanocrystal particles based upon this scheme.

In some embodiments, processing conditions can be adjusted or selectedto allow for the formation of nanocrystal particles such as tetrapods,bipods, tripods, branched tetrapods, and inorganic dendrimers.Processing conditions can be adjusted or selected to favor the growth ofone crystal structure over another to grow cores or arms as desired. Forexample, for compound semiconductors such as CdSe and CdTe, low reactiontemperatures favor the formation of a cubic crystal structure, whilehigher reaction temperatures favor the formation of a hexagonal crystalstructure. Low monomer concentrations favor the formation of a cubiccrystal structure, while high monomer concentrations favor the formationof a hexagonal crystal structure. Parameters such as these can bemanipulated to favor the growth of one crystal structure over another.For example, processing conditions can be selected so that they arefavorable to the formation of a cubic crystal core structure. Then, theycan be adjusted so that they are favorable to the formation of hexagonalcrystal arm structures. As will be explained in detail below, using suchmethods, polytypic nanocrystal particles can be formed.

A tetrapod shaped nanocrystal particle 10 according to an embodiment ofthe invention is shown in FIG. 1 and has a zinc blende core 12 withfour±{111} facets 18, each projecting a wurtzite arm 14(a), 14(b),14(c), 14(d) that is terminated with the ±(000 1) facet 16. In FIG. 1,the exploded view of one arm 14(a) illustrates the identical nature ofthe ±(111) zinc blende (ZB) and ±(000 1) wurtzite (WZ) facets of thecore and the arms, respectively. Phosphonic acid molecules 16selectively bind to the lateral facets of the arms 14(a)-14(d), assuggested in the figure (for clarity, only two facets are shown covered)to inhibit growth on these facets. High-resolution TEM analysis wouldfurther clarify the shape of the cubic nucleus and the relativeorientations between the various arms of the tetrapod.

Although CdSe and CdTe branched nanocrystal particles are described indetail herein, the branched nanocrystal particles may comprise anymaterial. For example, the particles may comprise semiconductors such ascompound semiconductors. Suitable compound semiconductors include GroupII-VI semiconducting compounds such as MgS, MgSe, MgTe, CaS, CaSe, CaTe,SrS, SrSe, SrTe, BaS, BaSe, BaTe, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS,HgSe, and HgTe. Other suitable compound semiconductors include GroupIII-V semiconductors such as GaAs, GaP, GaAs—P, GaSb, InAs, InP, InSb,AlAs, AlP, and AlSb. The use of Group IV semiconductors such asgermanium or silicon may also be feasible under certain conditions. Inother embodiments, the particles may comprise a dielectric material suchas SiC, SiN or any other material that can exhibit polytypism. Somemetals such as Fe, Ni, Cu, Ag, Au, Pd, Pt, Co and others may alsoexhibit polytypism and can be used in embodiments of the invention.

A. Methods for Forming Branched Nanocrystal Particles

Embodiments of the invention include methods of forming branched,nanocrystal particles. In one embodiment, the method comprises forming acore having a first crystal structure in a solution. The core may have acubic crystal structure (e.g., a zinc blende structure) or any othersuitable crystal structure. Then, one or more arms can form from thecore, simultaneously or sequentially. The arms may have differentcrystal structures than the core. If a tetrapod is formed, the tetrapodmay have first, second, third, and fourth arms, each with a hexagonalcrystal structure extending from the core.

The types of precursors used to form the branched, nanocrystal particlesdepend on the particular nanocrystal particles to be formed. In someembodiments, the precursors used to synthesize the nanocrystal particlesinclude Group II, III, IV, V, and/or VI semiconductor precursors. Forexample, in embodiments of the invention, semiconductor nanocrystalparticles including a Group II-VI compound semiconductor can be thereaction product of at least one precursor containing a Group II metalcontaining precursor and at least one precursor containing a Group VIelement, or a precursor containing both a Group II and a Group VIelement. In other embodiments of the invention, semiconductornanocrystal particles including a Group III-V compound semiconductor canbe the reaction product of at least one precursor containing a Group IIIelement and at least one precursor containing a Group V element, or aprecursor containing both a Group III and a Group V element. Otherexemplary precursors, surfactants, and solvents can be found in U.S.Pat. Nos. 6,225,198 and 6,306,736. These U.S. patents are hereinincorporated by reference in their entirety for all purposes.

If Group III-V semiconductor nanocrystal particles are to besynthesized, a Group III precursor, such as elemental Ga, In, Al, or anycompound containing a Group III precursor, such as a GaIII salt, InIIIsalt, or AlIII salt (e.g., of a halide, or corresponding metal-carbontrialkyls) can be reacted directly with an arsenic, phosphorus, orantimony source such as arsine, phosphine, or stibine; an alkyl arsine,phosphine or stibine; or an alkyl silyl arsine, phosphine or stibine inliquid phase at an elevated temperature. Representative metal sourcesinclude GaCl₃, GaBr₃, GaI₃, InCl₃, InBr₃, AlCl₃, Ga(Me)₃, Ga(Et)₃,Ga(Bu)₃, or the like. Representative arsenic, phosphorus and seleniumsources include AsH₃, PH₃, SeH₃, AsH₂ (carbon alkyl), As(carbon alkyl)₃,P(carbon alkyl)₃, As(Si(carbon alkyl)₃)₃, P(Si(carbon alkyl)₃)₃,Se(Si(carbon alkyl)₃)₃ and the like. Although specific examples ofprecursors are provided, any Group III or V element and any compoundcontaining such an element can be used in embodiments of the invention.

If Group II-VI semiconductor nanocrystal particles are to besynthesized, they may be the product of a reaction containing at leastone precursor comprising a Group II element such as Zn, Cd, or Hg, orany Group II containing molecule such as a metal, salt, oxide,organometallic compound, and at least one precursor comprising a GroupVI element such as O, S, Se, or Te, or any Group VI containing moleculesuch as a metal, salt, oxide, organometallic compound, or a precursorcontaining both a Group II element (Zn, Cd, or Hg) and a Group VIelement (S, Se, or Te). Those of skill in the art can select theappropriate precursors to form the appropriate compound semiconductor.For example, Cd(CH₃) and Se:TOP are examples of precursors respectivelycontaining Group II and Group VI elements that can be used to form CdSenanocrystal particles.

The precursors may be dissolved in any liquid compatible with thesurfactant mixture. Examples of organic liquids include polar organicsolvents including trialkyl phosphine, e.g., tributyl phosphine. In someembodiments, the precursors may be dissolved in the same solvent or maybe dissolved separately to form two or more precursor solutions.

Embodiments of the invention can use a surfactant mixture to make thesemiconductor nanocrystal particles. The surfactant mixture can be ahigh boiling point liquid mixture of two or more reactive ornon-reactive organic surfactants. The mixture of these organicsurfactants is capable of promoting the growth of branched semiconductornanocrystal particles.

The surfactant mixture can have a boiling point that is high enough sothat a reaction between, for example, the Group II and Group VIprecursors, or the Group III and Group V precursors, can take place toform the desired semiconductor nanocrystal particles. For example, insome embodiments, the surfactant mixture can have a boiling pointbetween about 200° C. to about 400° C.

The surfactant mixture may include any suitable number of differentsurfactants. For example, the surfactant mixture may include a firstorganic surfactant and a second organic surfactant. Third, fourth,fifth, etc. surfactants could also be used. For example, in someembodiments of the invention, at least one or two of the surfactants canbe selected from the group consisting of a phosphonic acid,trioctylphosphine oxide, an amine, oleaic acid, and stearic acid. Asnoted above, the surfactant mixture can be capable of being heated to acrystal-growing temperature, and can promote the growth of branchedsemiconductor nanocrystal particles such as tetrapods.

The first surfactant in the surfactant mixture may comprise aphosphorus-containing surfactant capable of withstanding suchcrystal-growing temperatures. Examples of such firstphosphorus-containing liquid surfactants include liquid surfactants suchas 3-30 (or larger) carbon trialkyl phosphines (e.g., tributylphosphine), or 3-30 or larger carbon trialkyl phosphine oxides (e.g.,trioctyl phosphine oxide or “TOPO”). The first surfactant may alsoinclude other surfactants including molecules including functionalgroups such as amines, carboxylic acids, and any other groups as long asthey are stable.

The surfactant mixture can include a second organic surfactant. Thesecond organic surfactant may be capable of being heated tocrystal-growing temperatures and may be capable of promoting the growthof branched semiconductor nanocrystal particles. Preferably, the secondliquid surfactant capable of promoting the growth of branchedsemiconductor nanocrystal particles can comprise a phosphorus-containingsurfactant capable of withstanding such crystal-growing temperatures.

The second organic surfactant may comprise an organic-substituted acid,or acid salt surfactant containing phosphorus such as, for example,phosphonic and phosphinic acids. Suitable phosphinic acids may includemono and diphosphinic acids having the general formulaR′R_(x)H_((1-x))POOH, where R and R′ are the same or different 3-30carbon (but preferably 3-30 carbon) organic groups such as alkyl or arylgroups, and x is 0-1. In some embodiments, the second organic surfactantcomprises a 3-30 carbon alkyl phosphonic acid, e.g., octadecylphosphonic acid.

The second organic surfactant is preferably a long chain lengthphosphonic acid. Short chain length phosphonic acids are defined asthose having an alkyl chain length of less than or equal to about 10carbon atoms. Long chain length phosphonic acids are defined as thosehaving an alkyl chain length of greater than or equal to about 10 carbonatoms. In preferred embodiments, the phosphonic acid is at least 14carbon atoms long. An example is octyldecylphosphonic acid (ODPA). Formaterials such as CdTe, these long chain length phosphonic acids help topromote the growth of hexagonal crystals.

In embodiments of the invention, a solution of one or more precursorscan be slowly and/or quickly injected into a heated surfactant mixture.Injecting precursors slowly is a relative term that is readilydeterminable by one having ordinary skill in the art. It can includeadding precursors drop by drop or no faster than 10 drops/sec, 5drops/sec, 2 drops/sec, or 1 drop/sec. Injecting precursors quickly isalso a relative term readily determinable by one having ordinary skillin the art. It can include adding precursors at a speed greater than 100drops/sec, 20 drops/sec, or 10 drops/sec. For example, injectingprecursors quickly can include emptying a 5 mL syringe holding theprecursor as quickly as possible.

A solution of precursors can be injected into the surfactant mixture ata cold or low temperature solution so that immediately after theinjection, the temperature of the hot mixture of surfactants drops to asecond, lower temperature. Initially, the heated surfactant mixture mayor may not contain other precursors. A pipette or a pressure nozzle maybe used as an injection apparatus. The temperature can be kept constantduring the nanocrystal growth. The resulting mixture is maintained at afirst temperature, which results in the nucleation of seed crystals.

It is understood that the different precursors could be in their ownseparate solutions and these different solutions could be separatelyinjected into the heated surfactant mixture in embodiments of theinvention. For example, if CdSe nanocrystal particles are to be formed,a Cd precursor solution and a Se precursor solution can be separatelyand sequentially injected into a hot surfactant mixture to producebranched CdSe nanocrystal particles. The separate injection ofprecursors into a hot surfactant mixture is preferred as it results inbetter control of the reaction, which can allow a higher percentage ofbranched nanocrystal particles if desired (e.g., a higher percentage oftetrapods).

The precise reaction time may vary depending on the particular materialused and the particular type of nanocrystal particles formed. In someembodiments, a 5 minute reaction time may be sufficient, while less thanor more than 5 minutes may be desirable in other embodiments.

Subsequent nanocrystal growth can then stopped by a further reduction ofthe temperature to below the temperature at which nanocrystal growthoccurs. Cessation of the crystal growth may be accomplished by rapidlyreducing the temperature to ambient temperature or even lower, e.g., toless than 150, 100, 75, 50, or 25° C. or lower, e.g., by removing theheating mantle that was used as a heat source. The temperature can bereduced more rapidly if the solution is cooled with a stream of air,cold water, liquid nitrogen, dry ice or other cooling agent.

After they are formed, the semiconductor nanocrystal particles can beseparated from the liquid medium that is used to form them. In someembodiments, a solvent such as methanol or acetone is added to theliquid medium containing the semiconductor nanocrystal particles toprecipitate them. For example, CdSe particles are generally not solublein polar solvents such as methanol or acetone. Any appropriate solventcan be added to precipitate the nanocrystal particles from the solution.

After the nanocrystal particles have precipitated, the precipitatednanocrystal particles are separated from the rest of the solution. Insome embodiments, centrifuging can be used to separate the nanocrystalparticles from other solution components. After centrifuging, thesupernatant can be separated from the nanocrystal particles. Thenanocrystal particles can then be stored as precipitate or can be driedin a vacuum.

In a typical synthesis of CdTe tetrapods, a mixture ofoctadecylphosphonic acid (ODPA), trioctylphosphine oxide (TOPO) andcadmium oxide (CdO) is degassed at 120° C. for 20 minutes and is slowlyheated under Ar until the CdO decomposes and the solution turns clearand colorless. Next, 1.5 g of trioctyl phosphine (TOP) is added, and thetemperature is further raised to 320° C. After that, Te:TOP(concentration of Te 10% by weight) is quickly injected into thesolution. The temperature drops to 315° C. and is maintained at thisvalue throughout the 5 minute synthesis. The resulting tetrapods areobtained in high yield, and they are soluble in common organic solvents,such as toluene and chloroform.

In some embodiments, branched nanocrystal particles such as tetrapodsmay be co-formed with other types of nanocrystal particles such as rodsor spheres. If this occurs, it is possible to separate the tetrapodsfrom the rods to increase the percentage of tetrapods in a given sample.For example, a solution may contain nanocrystal rods and tetrapods. Thetetrapods can constitute more than about 1%, 5%, 10%, 20%, 40%, 60%,80%, 90%, 95%, or 99% of all particles in the initial sample dependingon the growth conditions. Processes for separating tetrapods from othernanocrystals are described in the Examples below.

Because the branched nanocrystal particles according to embodiments ofthe invention can be formed in a surfactant mixture, the nanocrystalparticles that result may be functionalized with surfactant molecules.For example, the core, and/or arms of a nanocrystal particle accordingto embodiments of the invention may be functionalized with any of theabove-mentioned surfactant molecules (e.g., amphiphilic molecules). Itis noted that the arrow and teardrop shaped nanocrystal particlesdescribed below could also be functionalized with surfactant molecules.

The branched nanocrystal particles according to embodiments of theinvention can be formed with arms of any suitable length or diameter. Itis possible to control the phase during nucleation and growth, and tomanipulate the growth kinetics of the nanocrystal particles to enableindependent tuning of the arm lengths and diameters. The presentinventors have found that once the basic tetrapod shape is formed,growth of the arms occurs according to the controllable kineticmechanisms previously observed for nanorods. See, for example, Manna etal., J. Am. Chem. Soc. 2000, 122, 12700-12706.

For example, with respect to CdTe tetrapods, higher Cd/Te ratios resultin longer arms, while more phosphonic acid per Cd yields larger armdiameters. Anisotropy results from fast growth, and the growth rate islimited by the concentration of the Cd precursor, which is a strongcomplex between Cd²⁺ and phosphonic acid. Hence, higher Cd/Te ratioskeep the reaction in the anisotropic growth regime longer, leading tolonger arms. On the other hand, the presence of more phosphonic acid perCd (lower Cd/ODPA ratio) likely decreases the diffusion constant of theCd precursor and the driving force for its addition to the crystal,thereby slowing the growth rate for a given Cd concentration. However,the growth of the arms continues as long as Cd concentration issufficiently high. This results in less anisotropic rods, with a largerdiameter for a given length.

Tetrapods with arms of identical length can also be formed. Thisgenerally involves the simultaneous development of the four wurtzitearms and a highly homogeneous environment around the growing tetrapodstructure. This implies, for instance, a fast and isotropic supply ofmonomer species from the bulk. When these homogenous conditions are notpresent, some arms can grow substantially slower than others, or evennot at all, resulting in so-called “tripods”, “bipods” and “monopods”.Tripods and bipods are examples of branched nanocrystal particles.Additionally, missing arms can be due to differences in the shrinkagerate of the various arms in the same tetrapod.

The nanocrystal particles according to embodiments of the invention canhave a variety of interesting mechanical, electrical, and opticalproperties. For example, due to their three dimensional character,tetrapods may be important alternatives to nanocrystal fibers and rodsas additives for mechanical reinforcement of polymers (e.g., polymericbinders including polyethylene, polypropylene, epoxy functional resins,etc.). Tetrapod shaped nanocrystal particles, for example, can interlockwith each other and can serve as a better reinforcing filler in acomposite material (e.g., with a binder), than for example, nanospheres.The nanocrystal particles can be mixed with the binder using anysuitable mixing apparatus. After the composite material is formed, thecomposite material can be coated on a substrate, shaped, or furtherprocessed in any suitable manner.

The nanocrystal particles according to embodiments of the invention canalso have unique optical properties. For example, FIGS. 2( a) and 2(b)compare the electronic absorption spectra for two series of tetrapodsamples having different arm lengths and diameters. In a tetrapod shapednanocrystal, most of the confinement energy is along the diameter of thehexagonal arms. Tetrapods having comparable arm lengths but differentdiameters, show remarkable differences in their band gap energy (FIG. 2(a)). While spectra of tetrapods with comparable diameters but differentarm lengths, are almost identical (FIG. 2( b)). This independenttenability of the arm length and the band gap is very attractive fornanocrystal based solar cells or other types of photovoltaic devices.Exemplary photovoltaic devices are described in, for example, Science,Vol. 295, pp. 2425-2427, Mar. 29, 2002. An exemplary photovoltaic devicemay have nanocrystal particles in a binder. This combination can then besandwiched between two electrodes (e.g., an aluminum electrode and anindium tin oxide electrode) on a substrate to form a photovoltaicdevice.

The inherent property of a tetrapod to self-align on a substrate withone arm always pointing towards one electrode, combined with the lowband gap material such as CdTe, should substantially enhance the deviceefficiencies of recently reported hybrid nanorod-polymer solar cells. Incomparison to nanocrystal particles that are randomly oriented, thetetrapods according to embodiments of the invention are aligned and canprovide for a more unidirectional current path than randomly orientednanocrystal particles.

Although tetrapods have just been described in detail, it is understoodthat embodiments of the invention include even more complex shapednanocrystal particles. In embodiments of the invention, the initialnucleation event yields a core with a cubic crystal structure (e.g., azinc blende crystal structure). Later, arms with a hexagonal crystalstructure (e.g., wurtzite) can grow out from the core. However,different growth conditions can be provided to statistically alternatethe formation of cubic and hexagonal crystal structures, thus leading toirregular branching. Precise control of temperatures throughout thereaction may yield sequentially branched “inorganic dendrimers”. Thiscan be illustrated with reference to FIGS. 3( a)-3(c).

FIG. 3( a) shows a tetrapod 300 according to an embodiment of theinvention looking down one arm of the tetrapod 300. The tetrapod 300includes a cubic core 102(a) and four arms with hexagonal crystalstructures extending from the cubic core 102(a). Adjacent arms can formangles of about 109.5 degrees (it has tetrahedral symmetry) in someembodiments. In FIG. 3( a), three arms 104(a)-104(c) are shown, with thefourth arm of the tetrapod extending toward the reader. As shown, eacharm can include a proximate end and a distal end. For example, the arm104(a) has an end 104(a)-1 that is proximate to the core 102(a) and anend 104(a)-2 that is distal to the core 102(a).

In the formation of the tetrapod, the cubic core 102(a) forms and thenthe four arms having a hexagonal crystal structure grow from that core102(a). Processing conditions can be adjusted to cause the arms to growfrom the core 102(a) or the arms may inherently form from a given set ofconditions. For example, for CdTe nanocrystal particles, providinghexagonal crystal growing conditions can result in tetrapods without theneed to adjust processing conditions. As the present inventors havedemonstrated, simply providing conditions favorable hexagonal crystalgrowth can result in CdTe tetrapods. Alternatively, as shown in the CdSetetrapod examples below, processing conditions can be adjusted to formbranches. Higher monomer concentrations (e.g., adding more precursor toa surfactant mixture) and higher temperatures can be used to induce theformation of arms with hexagonal crystal structures, while lower monomerconcentrations and lower temperatures can be used to induce theformation of cubic crystal structures.

With this in mind and referring to FIG. 3( b), once the basic tetrapodis formed, additional branches on the tetrapod can be formed. Forexample, as shown in FIG. 3( b), additional arms 106(a), 106(b) can formfrom a second core 102(b) at the distal end 104(a)-2 of the arm 104(a).The arms 106(a), 106(b) may appear as branches. As a result, anothertetrapod can form at the distal end 104(a)-2 of the arm 104(a). Theresulting nanocrystal particle may be a branched tetrapod 301.

To form the branched tetrapod 301 in FIG. 3( b), an initial tetrapod 300can first be formed as shown in FIG. 3( a). Growth conditions will favorthe formation of the hexagonal arms 104(a)-104(c). Then, the processingconditions can be adjusted to form cores at the ends of the arms104(a)-104(c). For example, the processing temperature can be loweredand/or the monomer concentration can be lowered. These conditions, ingeneral, result in the formation of additional (or second) cubic crystalstructure cores 102(b)-102(d) at the ends of the arms 104(a)-104(c).Once the additional cubic crystal structure cores are formed, processingconditions can again be adjusted to favor the growth of hexagonalstructure arms. For example, the processing temperature can be raisedand/or the monomer concentration in the solution can be increased tocause arms with hexagonal crystal structures to form. This process canbe repeated as often as desired to form, for example, an inorganicdendrimer 303. An example of an inorganic dendrimer 303 is shown in FIG.3( c). Compared to the tetrapod 300 and the branched tetrapod 301, theinorganic dendrimer 303 is more complex in shape.

Other schematic, three-dimensional illustrations of how branchedtetrapods can form are in FIGS. 4( a)-4(c). In FIGS. 3( a)-3(c) and4(a)-4(c), like numerals designate like elements and the description ofcommon elements need not be repeated.

FIG. 4( a) shows a tetrapod 300. As noted above, as shown in FIG. 4( b),processing conditions can be selected so that four cores 102(b)-102(e)form at the distal ends of four arms of a basic tetrapod. In FIG. 4( b),the faces 203(a), 203(b), 203(c) of the core 102(b) are more clearlyshown. As shown in FIG. 4( c), arms 106(a)-106(b) form on the faces203(a)-(c). For clarity, only two arms 106(a)-106(b) are shown in FIG.4( c).

II. Arrows

In other embodiments of the invention, arrow-shaped nanocrystalparticles can be formed. In an exemplary embodiment, precursors areintroduced into a mixture of surfactants to form a solution. As notedabove, the mixture of surfactants can be hot and the precursors can beintroduced using an injection process. Once the precursors and thesurfactants are mixed together, arrow-shaped nanocrystal particles canform in the solution. It is understood that “arrow-shaped” nanocrystalparticles can include tree-shaped nanocrystal particles such aspine-tree shaped nanocrystal particles.

Examples of precursors, solvents, surfactants, and processing conditions(e.g., injection rates, processing temperatures, etc.) are describedabove, and need not be repeated herein. Any of these may be suitable foruse with the formation of arrow-shaped nanocrystal particles. However,to form arrow-shaped nanocrystal particles, the amount of the secondsurfactant is preferably greater than about 30 mole %, 60 mole %, andeven 70 mole % based on the total moles of the surfactants in thesurfactant mixture (e.g., based on the total moles of the first andsecond surfactants). Surprisingly and unexpectedly, higherconcentrations of the above described second surfactant can result inarrow-shaped nanocrystal particles.

In an exemplary embodiment, to form CdSe arrow-shaped nanocrystalparticles, cadmium and selenium precursors are co-dissolved intri-n-butyl phosphine. This precursor solution is manually injected into4 grams of a hot (360° C.) binary surfactant mixture of TOPO and HPA.The amount of HPA is 60 mole % based on the total moles of TOPO and HPA.Nanocrystal particles form and some of these nanocrystal particles arein the form of arrows. The arrows are precipitated and separated fromother particles as described in the Examples section below.

Higher HPA ratios naturally lead to the formation of arrow shapednanocrystal particles. Without being bound by theory, this can beunderstood as arising from further enhancement of the growth rate of the(00 1) face relative to the other faces. In crystal growth, the fastestgrowing face is eventually replaced by slower growing faces, and this ishow the basic arrow shapes (see, e.g., the arrows in FIGS. 6( b)-6(e))form. The {101} faces of the arrow grow more slowly than the (00 1)face, and in the high HPA limit where (00 1) is growing extremelyquickly, it is eventually replaced by the (101) equivalent faces. Withinthe kinetic regime, these variations of shape will arise just fromdifferential growth rates of the various faces, regardless of absoluterate of growth (whether the growth of the unique face is enhanced or thegrowth rates of all the other faces are retarded by the HPA).

Using TEM images of samples taken at different times after injection, itis possible to follow the shape evolution as the reaction proceeds. Thepercentage of narrow arrow and tree shaped nanocrystal particles (whichcan also can be considered arrows) increases with time while the amountof rods and pencils decreases. Despite the fact that the nanocrystalparticles presumably grow from rods to pencils to arrows (and to a morespecific form of arrow, a pine tree), the rate of growth of the (00 1)face is nearly constant. The average lengths of different particleshapes at a certain time can be within about 2% of each other,indicating that the rate of growth along the c-axis is not affected bythe additional growth on the sides of the nanocrystal particle.

The formation of arrows suggests unidirectional growth. The hexagonalCdSe nanocrystal particles do not have inversion symmetry, meaning thetop and bottom faces of the crystals are intrinsically different. As canbeen seen in FIG. 10, for example, Cd atoms on the (001) face have onedangling bond, while Cd atoms on the (00 1) face have three danglingbonds. It appears that in the presence of HPA, the relative growth rateof the (00 1) face is much greater than that of the others.

III. Teardrops

Other embodiments of the invention are directed to teardrop-shapednanocrystal particles. Teardrop shaped nanocrystal particles can beformed by mixing semiconductor precursors and a mixture of surfactantsto form a solution. Examples of precursors, solvents, surfactants, andprocessing conditions (e.g., injection rates, processing temperatures,etc.) are described above, and need not be repeated herein. Any of thesemay be suitable for use with the formation of teardrop-shapednanocrystal particles.

To form teardrop shaped nanocrystal particles, however, a first amountof one or more semiconductor precursors are injected into a hotsurfactant mixture to form a precursor/surfactant solution. A slowinjection rate (1.0 ml/s) results in a drop of the monomer concentrationbelow the Ostwald ripening limit for some time (e.g., into about 4 gramsof surfactant mixture). After this first amount is introduced, thereaction proceeds and there is a waiting period where no precursor isintroduced to the solution. This waiting time may vary depending on theexact teardrop morphology desired. For example, the waiting time betweensequential introductions of precursors into a hot surfactant mixture canbe as little as 30 seconds in some embodiments, and can be greater thanabout 1 minute in other embodiments. (As an alternative to waiting andnot introducing precursors into the hot surfactant mixture betweensuccessive precursor injections, the rate of introduction of theprecursors can be reduced relative to prior rates of precursorintroduction.) At this point the nanocrystal particles will form lowaspect ratio rods, or if left under these conditions for a considerableamount of time, nearly spherical or oblong dots. After a predeterminedamount of time, a second amount of the precursors is introduced into thesolution (or the rate of precursor introduction into the surfactantmixture can be increased relative to a prior precursor introductionrate). The monomer concentration is once again increased with anadditional slow injection to reinitiate rod growth. The increase in themonomer concentration causes the somewhat spherical or oblongnanocrystal particles to form elongated portions. The resultingnanocrystal particles are teardrop shaped.

In an exemplary embodiment, to form teardrop particles, a 1.0 ml ofstock solution including Cd and Se precursors can be injected into 20%HPA in TOPO at 360° C. at a rate of about 10 ml/s. The temperature canbe maintained at 328° C. The high temperature and low monomerconcentration promotes Ostwald ripening of the nanocrystal particles. Anadditional slow injection of 2.0 ml of the same stock is made after oneminute. This injection can take about 4 minutes. After about 20 minutes,after the second injection, the synthesis is stopped. Teardrop shapednanocrystal particles are formed after the second injection.

Teardrop growth illustrates how time varying concentrations can be usedto create nanocrystal particles with complex shapes. The unidirectionalgrowth noted in arrow formation is also a factor in the formation of theteardrops. Here, however, it is possible to take advantage of a thirdmajor effect, namely, that slow growth favors equilibrium, and roundshapes. Teardrop shapes arise when rod like crystals are subsequentlygrown at low monomer concentration and slow injection volume. Then, themonomer concentration is abruptly increased to cause the teardrop toelongate. Put another way, rods are first formed and then they becomerounded (e.g., due to ripening), forming the body of the tear. Then,when the monomer concentration increases, the droplet elongates.

More specific examples of embodiments of the invention can be describedwith respect to the foregoing examples.

III. Examples

A. Synthesis of CdSe Nanocrystal Particles

Dimethylcadmium (Cd(CH₃)₂, 97%) and tri-n-butylphosphine (C₁₂H₂₇P orTBP, 99%) were purchased from Strem. Cd(CH₃)₂ was vacuum transferred andstored at −35° C. under argon. Selenium (Se) (99.999%),tri-n-octylphosphine oxide (C₂₄H₅₁OP or TOPO, 99%), and hexylphosphonicdichloride (C₆H₁₃Cl₂OP, 95%) were purchased from Aldrich. All solventsused were anhydrous, purchased from Aldrich and used without any furtherpurification. Hexylphosphonic acid (C₆H₁₅O₃P or HPA) was prepared fromhexylphosphonic dichloride according to a standard procedure(Andriano.Ka et al., Zhurnal Obshchei Khimii, 40:1565-& (1970)).

All manipulations were performed using standard air-free techniques. Forthe synthesis of CdSe nanocrystal particles, cadmium and seleniumprecursors were co-dissolved in tri-n-butyl phosphine and the resultingstock solution was stored in a refrigerator at −20° C. The solution wasquickly removed from the refrigerator and vigorously agitated for 10seconds. It was then manually injected, under Ar, via a syringe, into 4grams of a hot (360° C.) binary surfactant mixture of TOPO and HPA.Unless otherwise stated, the above techniques were used in all of thefollowing syntheses.

Various stock solutions were all made in a glove box under Ar. For stocksolution A (molar ratio of Cd:Se of 1.4:1), 0.82 g Cd(CH₃)₂, 1.6 gSe:TBP (20% Se by weight) and 14.08 g TBP were stirred for 5 minutes andthen placed in a refrigerator at −20° C. Stock solution B had a ratio ofCd:Se of 1:1 and was made from 0.82 g Cd(CH₃)₂, 2.27 g Se:TBP (20% Se byweight), and 13.41 g TBP. Stock solution C had a ratio of Cd:Se of 1.9:1and was made from 0.82 g Cd(CH₃)₂, 1.20 g Se:TBP (20% Se by weight), and14.48 g TBP. These stock solutions were used to create tetrapod,branched tetrapod, teardrop, and arrow shaped nanocrystal particles.

1. Surfactant Ratio Experiments

For the low HPA concentration experiment, 3.88 g of TOPO and 0.12 g HPAwere mixed in a 3-neck flask under Ar, and then heated to 360° C. withconstant stirring. This mixture is 3% HPA by weight and 8% by molarconcentration. The medium concentration experiment was 8% HPA by weight(20% molar), and used 3.68 g TOPO and 0.32 g HPA. The high concentrationexperiment was 20% HPA by weight (58% molar), and used 3.20 g TOPO and0.80 g HPA. Henceforth, all concentrations of HPA in TOPO are given inmolar concentrations (based on the total amount of HPA and TOPOsurfactant), unless otherwise stated. For each of these experiments, 2.0ml of stock solution A was injected into solution at a rate ofapproximately 20 ml/s. Aliquots were taken at 4, 10 and 30 minutes afterthe injection. The reactions were stopped after 30 minutes by quenchingthe solution with toluene. These experiments were repeated three timeseach. The temperature drop observed during rod experiments was from 360°C. to ˜300° C. and the temperature was maintained at 300° C. unlessotherwise stated.

2. Ripening Experiment

To form teardrop-shaped nanocrystal particles, a 1.0 ml of stocksolution of A was injected into 20% HPA in TOPO at 360° C. at a rate of10 ml/s. The temperature was kept at 328° C. The high temperature andlow monomer concentration promoted Ostwald ripening of the nanocrystalparticles. An additional slow injection of 2.0 ml of stock solution Awas started after one minute. This injection took 4 minutes. Thesynthesis was stopped 20 minutes after completing the slow injection.

3. Shape Selective Dissolution and Precipitation

When the syntheses described above yielded monodisperse samples (bothsize and shape), no further size selection was applied. If adistribution of lengths and shapes was observed, the following procedurewas used to separate them. Methanol was added to the nanocrystalsolution until the nanocrystal particles all precipitated. Thisprecipitate was washed twice with methanol to remove residual TOPO, TBPand HPA and was redissolved in toluene. This solution was centrifugedfor 30 minutes. If a precipitate (1) appeared at the bottom of the vial,the supernatant (2) was transferred in another vial and the precipitate(1) was dissolved in chloroform. This solution (1) contained the longestrods. In the case of long rods (40 nm or longer), not all theprecipitate was soluble in chloroform and the dispersion became clearafter the addition of a small amount of dodecylamine (1-2 mg for a 100mg precipitate). To the supernatant (2), methanol was added drop-wisewhile under constant agitation, until the solution became cloudy. Thesolution was then centrifuged and the precipitate (3) was dissolved intoluene or chloroform. This procedure was repeated obtaining tetrapodsand short rods in each subsequent precipitate. In all of the abovecases, the final product is filtered through a 0.2 um PTFE filter toremove any non-nanoscale materials that might be present.

4. Characterization of Samples.

a. UV-Vis Absorption Spectroscopy

Absorption spectra were created using a Hewlett Packard 8453 UV-visiblediode array spectrometer equipped with a deuterium lamp having aresolution of 2.0 nm. A small amount of sample (˜10 μl) was removed viasyringe and diluted to an optical density of between 0.1 and 0.5 byaddition of either toluene or chloroform. The exciton peak in theabsorption spectrum taken immediately after injection is broad andbetween 600-620 nm. If monitored throughout the growth, the exciton peakblue-shifts to around 560 nm and then narrows. This happens because uponinjection, there is a broad size distribution of rod lengths. As therods grow longer, the long axis grows beyond the confinement regime, andthe exciton peak only depends on the short axis (diameter) of the rods.Unlike typical nanocrystal syntheses where the peak red-shifts as thesize increases, the rods blue-shift. As the length increases beyond theconfinement regime, the exciton peak is only dependent on the shortaxis. The short axis (3-4 nm) is smaller than the rods were initiallylong, so the exciton peak blue shifts even though the rods increase inlength.

b. Transmission Electron Microscopy

Nanocrystal size, morphology and structure were measured by TEM at theNational Center for Electron Microscopy at Lawrence Berkeley Laboratory,on a Topcon EM002B electron microscope. The microscope was operated atan accelerating voltage of 120 kV to minimize beam damage to the sample.

Nanocrystal particles were deposited from dilute solution onto a 3-4 nmthick film of amorphous carbon supported by 400 mesh copper grids. Onedrop of nanocrystal solution in either toluene or chloroform wasdeposited onto the grid and allowed to evaporate. The sample was thenwashed with methanol to remove excess organic compounds and placed in avacuum dessicator overnight.

Structural determination and observation of stacking faults wasaccomplished using high resolution TEM (HRTEM) at 440,000 timesmagnification. Average sizes and morphologies were measured at 88,000times magnification, calibrated using known crystal lattice spacingsmeasured at higher magnifications. Average lengths and shapedistributions were determined by counting at least 300 nanocrystalparticle's per sample for statistical purposes.

c. Powder X-Ray Diffraction

Powder X-ray diffraction was performed on a Bruker-AXS D8 general areadetector diffraction system (GADDS), using Co Kα radiation (1.79026 Å).Two-dimensional patterns were angle integrated to obtain the patternsdisplayed. The instrument resolution is 0.07° in 2θ and the accumulationtime for each sample was at least 20 minutes. The 2θ range used was from20°-65° (Q=1.5-4.0 Å⁻¹, Q=(4π sin θ)/λ) at an Ω angle of 15°. XRDsamples were prepared by evaporating several drops of a nanocrystalsolution on a quartz plate. Prior to the measurements, the samples werewashed with methanol to remove excess organic material and dried.

XRD sizing of particles was performed using the Debye-Scherrer equation(Guinier, A. X-Ray Diffraction In Crystals, Imperfect Crystals, andAmorphous Bodies; Dover: N.Y., 1994). The (002) peak at Q=1.8 Å⁻¹ wasused to determine the length of the crystalline domain along the longaxis of the rods. Peaks were fit using commercial software (PeakFit™ v4)utilizing a Gaussian*Lorentzian peak shape. The instrument broadeningwas measured using bulk LaB₆, and then subtracted using a standardcorrection (Guinier, A. X-Ray Diffraction In Crystals, ImperfectCrystals, and Amorphous Bodies; Dover: N.Y., 1994) to the Debye-Scherrerequation.

5. Results

For fixed injection conditions, variations of the TOPO/HPA ratiosystematically controls the nanocrystal shape. This is clearly seen inthe low-resolution TEM image in FIG. 5 and in Table 1.

TABLE 1 HPA Injection Concentration Volume Length Aspect Ratio (molar)(ml) (nm) (c:a) 8% 2.0  5.1 ± 0.8 1:1 20% 2.0 21.8 ± 4.2 5:1 60% 2.021.7 ± 2.0 varies 20% 1.0 13.0 ± 2.1 2:1 20% 1.5 16.4 ± 1.1 2.7:1   20%2.0 21.8 ± 4.2 5:1

With no HPA and at low concentrations of HPA (less than 10%), roughlyspherical dots are formed. At HPA concentrations of 20%, high aspectratio rod growth is strongly favored. Finally, if the HPA concentrationis ˜60%, nanocrystal particles shaped like arrows are obtained (FIG. 5and FIG. 6( a)). The inventors observed nanocrystal particles thatlooked like narrow arrows (FIG. 5). The time dependence of the shapecomposition of the nanocrystal particles and the average lengths isgiven in Table 2.

TABLE 2 Time (min) Length (nm) Rod/Pencil Arrow 4 21.7 ± 2.0 56% 44% 1027.4 ± 3.0 37% 63% 30 31.6 ± 3.4 35% 65%

The amount of arrow shaped nanocrystal particles increases with time asthe amount of rod and pencil shaped nanocrystal particles decrease. Theaverage lengths of the long axis (c-axis) of the different shapes ofparticles within each sample are within 2% of each other.

6. Teardrops

From FIGS. 8( a)-8(b), it appears that particle growth occursselectively on one crystal face of the rods, thus forming almostteardrop-shaped particles. There are exceptions as can be seen in FIG.8( c), which shows a particle with growth occurring on two faces.Characterizing these cases with HRTEM revealed that they representnanocrystal particles with largely zinc blende structure, a defect, or acombination of the two. All crystals that were pure wurtzite grewsignificantly more on the (00 1) face of the nanocrystal, formingteardrops.

7. Tetrapods

Another nanocrystal shape that can be consistently obtained is tetrapodnanocrystal particles as seen in FIG. 9( a). There are lattice fringesthroughout the crystal, indicating crystallinity in both the center andthe arms. If tetrapods are observed in a synthesis that leads to rodformation, they can be selected out via size/shape selectiveprecipitation as detailed above. There is a relationship between aparticles size and shape, and its solubility. In general, the larger theparticle, the less soluble it is (assuming the same coating bysurfactant). If there is a mixture of tetrapods with arms as long as therods, and rods, then the tetrapods are less soluble and will precipitatebefore the rods.

When performing additional injections into solutions containingtetrapods, “dendritic” tetrapods can be formed as seen in FIG. 9( b). Upto three additional “branches” can be grown off the end of each arm ofthe original tetrapod.

As noted, tetrapods are single-crystal particles that demonstratepolytypism, having a tetrahedral zinc blende core and four wurtzitearms. Like the {001} planes of the wurtzite structure, the {111} planesof the zinc blende structure contain layers alternately composed ofeither Cd or Se as can be seen in FIG. 7. Since the presence of HPAselectively increases the growth rate of the (00 1) face, it followsthat the closely related zinc blende {111} faces would also grow quicklyin this binary surfactant mixture. The tetrapods are formed when a CdSenanocrystal nucleates in the zinc blende structure instead of thewurtzite structure. Then, wurtzite arms grow out of the four (111)equivalent faces of the tetrahedral zinc blende core as seen in FIG. 11.As noted above, there may be several ways of selectively adjusting therelative amounts of zinc blende versus wurtzite nuclei formed in theinjection process.

More complex shapes, such as dendritic tetrapods, are produced byperforming additional slow injections of monomer into a solution alreadycontaining tetrapods. Growth occurs at the ends of the tetrapod armswith each additional injection. If the arms are purely wurtzite, theywill continue to grow straight. If there are zinc blende layers orstacking faults near the end of the arms, which is a statisticalprobability, multiple (up to 3 maximum) additional “branches” will growout of each arm. This can be clearly seen in FIG. 9( b), where a secondprecursor injection was been performed on a sample containing tetrapodsafter the monomer concentration was allowed to decrease therebyencouraging the formation of zinc blende layers at the end of thetetrapod arms.

B. Synthesis of CdTe Tetrapods

1. Materials

Cadmium oxide (CdO) (99.99+%), Tellurium (Te) (99.8%, 200 mesh), andtri-n-octylphosphine oxide (C24H51OP or TOPO, 99%) were purchased fromAldrich. n-Octadecylphosphonic acid (C18H39O3P or ODPA, 99%) waspurchased from Oryza Laboratories, Inc. Trioctylphosphine (TOP) (90%)was purchased from Fluka. All solvents used were anhydrous, purchasedfrom Aldrich, and used without any further purification.

2. Synthesis of CdTe Tetrapods

All manipulations were performed using standard air-free techniques. TheCd/Te molar ratio was varied from 1:1 to 5:1, and the Cd/ODPA molarratio was varied from 1:2 to 1:5. The Te precursor solution was preparedby dissolving tellurium powder in TOP (concentration of Te 10 wt. %).The mixture was stirred for 30 minutes at 250° C. then cooled andcentrifuged to remove any remaining insoluble particles. In a typicalsynthesis of CdTe tetrapods, a mixture of ODPA, TOPO, and CdO wasdegassed at 120° C. for 20 minutes in a 50 ml three-neck flask connectedto a Liebig condenser. It was heated slowly under Ar until the CdOdecomposed and the solution turned clear and colorless. Next, 1.5 g oftrioctyl phosphine (TOP) was added and the temperature was furtherraised to 320° C. After that, the Te:TOP precursor solution was injectedquickly. The temperature dropped to 315° C. and was maintained at thisvalue throughout the synthesis. All syntheses were stopped after 5minutes by removing the heating mantle and by rapidly cooling the flask.After cooling the solution to 70° C., 3-4 ml anhydrous toluene wereadded to the flask, and the dispersion was transferred to an Ar drybox.The minimum amount of anhydrous methanol, which was used to precipitatethe nanocrystal particles after centrifugation, was added to thedispersion. In this way, potential co-precipitation of theCd-phosphonate complex was prevented. After removing the supernatant,the precipitate was re-dissolved twice in toluene and re-precipitatedwith methanol. After removing the supernatant, the final precipitate wasstored in the drybox. All resulting CdTe tetrapods were readily solublein solvents such as chloroform or toluene.

3. Characterization of Samples by Transmission Electron Microscopy (TEM)and UV-Vis Absorption Spectroscopy.

The structure and size of the CdTe nanocrystal particles were measuredvia TEM. At the UC Berkeley Electron Microscope Lab, a FEI Tecnai 12electron microscope was used. The microscope was operated at anaccelerating voltage of 100 kV. To evaluate the growth kinetics of thesyntheses, a small amount of the sample (˜0.1 ml) was removed viasyringe from the flask every minute and mixed into anhydrous toluene.The aliquots were transferred to the drybox and washed once withmethanol. The precipitated nanocrystal particles were re-dissolved intoluene and deposited from dilute solution onto a 3-4 nm thick film ofamorphous carbon supported by 400 mesh copper grids. One drop ofnanocrystal solution in toluene was deposited onto the grid andevaporated. UV-Vis absorption spectra were measured using aHewlett-Packard 8453 UV-visible diode array spectrometer equipped with adeuterium lamp having a resolution of 1.0 nm.

4. Table of Syntheses

Table 4 shows reagent amounts in CdTe tetrapod syntheses. (In order tocompensate for losses during injection, the recorded amount of Te:TOPslightly exceeds the one corresponding to the cited Cd/Te ratio.)

TABLE 4 Cd/Te molar ratio 1:1 2:1 3:1 5:1 Cd/ODPA 1:2 TOPO [g] 3.833.469 3.19 3.19 molar ODPA [g] 0.27 0.531 0.81 0.81 ratio CdO [g] 0.0510.102 0.153 0.153 Te:TOP [g] 0.540 0.540 0.540 0.350 1:3 TOPO [g] 3.733.20 3.19 3.19 ODPA [g] 0.27 0.80 0.81 0.81 CdO [g] 0.035 0.102 0.1020.102 Te:TOP [g] 0.400 0.540 0.400 0.240*⁾ 1:5 TOPO [g] 3.544 2.67 2.652.65 ODPA [g] 0.456 1.33 1.35 1.35 CdO [g] 0.035 0.102 0.102 0.102Te:TOP [g] 0.400 0.540 0.400 0.240*⁾ *⁾0.100 g TOP additionallyinjected.

Beyond control of the phase during nucleation and growth, manipulationof the growth kinetics enables independent tuning of the arm lengths anddiameters. FIG. 12 shows a series of transmission electron microscopy(TEM) images of typical CdTe tetrapods of various lengths and aspectratios, illustrating the influence of the main growth parameters onshape. The present inventors found that once the basic tetrapod shape isformed, growth of the arms occurs according to the controllable kineticmechanisms previously observed for nanorods.

In the CdTe tetrapod syntheses, the Cd/Te ratio was varied from 1:1 to5:1, and the Cd/ODPA ratio was varied from 1:2 to 1:5 (the Cd/OPDA ratiowas 1:2 and was at maximum in order for the CdO to decomposecompletely). An increase in the Cd/Te ratio leads to tetrapods withlonger arms, whereas higher Cd/ODPA ratios result in larger armdiameters. In all the experiments, the amount of Te:TOP solutioninjected was adjusted with respect to the Cd/Te ratio. Also, the amountof ODPA added varied depending on the Cd/OPDA ratio. The total amount ofTOPO+ODPA was always equal to 4 grams. For the syntheses done at Cd/ODPAratio of 1:2, the amount of CdO initially dissolved in the TOPO/ODPAmixture was 51 mg (1:1 Cd/Te), 102 mg (2:1 Cd/Te), and 153 mg (for both3:1 and 5:1 Cd/Te), respectively. For the syntheses done at Cd/ODPAratios of 1:3 and 1:5, the amounts of CdO initially dissolved in theTOPO/ODPA mixture are 35 mg (1:1 Cd/Te) and 102 mg (for 2:1, 3:1 and 5:1Cd/Te), respectively. Avoiding a large temperature drop after theinjection of the Te:TOP solution is desirable to ensure both a fasterrecovery of the thermal equilibrium between the flask and the heatingmantle and a higher homogeneity and reproducibility of the reactionconditions.

A further consequence of the kinetically controlled growth appears inthe shape evolution of tetrapods beyond the anisotropic growth regime.In FIG. 13, CdTe tetrapods extracted from the same synthesis, at 1 andat 5 minutes, respectively, for two syntheses carried out at the sameCd/Te ratio, but at two different Cd/ODPA ratios, are compared. In bothcases, most of the anisotropic growth takes place in the first minuteafter injection, when the concentration of monomers is high. The facetgrowing most quickly during this period is the one with the highestinterfacial energy. However, when the concentration of monomer drops,this facet is also the one that starts dissolving first. For instance,at high (1:2) Cd/ODPA ratio, the tetrapods grown for 5 minutes havedistinctly rounded ends. The dissolution of the end of an arm (the(000 1) facet) increases the local monomer concentration, allowing thelateral facets to grow at its expense, resulting in round, fat ends ofthe arms. This effect is not apparent in the sample grown at a lower(1:5) Cd/ODPA ratio, since slower growth rate delays the Ostwaldripening regime to longer times.

FIG. 14 shows a typical powder X-ray diffraction (XRD) of a CdTetetrapod sample (non-vertical line with peaks). The bulk XRD pattern ofCdTe wurtzite is also shown (vertical lines). The 002 peak is verynarrow and more intense than the other peaks because of the extendeddomain along the c axis of the tetrapod arms.

The terms and expressions which have been employed herein are used asterms of description and not of limitation, and there is no intention inthe use of such terms and expressions of excluding equivalents of thefeatures shown and described, or portions thereof, it being recognizedthat various modifications are possible within the scope of theinvention claimed. Moreover, any one or more features of any embodimentof the invention may be combined with any one or more other features ofany other embodiment of the invention, without departing from the scopeof the invention.

All patents, patent applications, and publications mentioned above areherein incorporated by reference in their entirety for all purposes.None of the patents, patent applications, and publications mentionedabove are admitted to be prior art.

What is claimed is:
 1. A method of forming a functional element of aphotovoltaic device comprising: forming a plurality ofsemiconductor-nanocrystals, wherein at least a portion of thesemiconductor-nanocrystals have an aspect ratio greater than about 5;and mixing the semiconductor-nanocrystals with a binder.
 2. The methodof claim 1, wherein the binder comprises a polymer.
 3. The method ofclaim 2, wherein the polymer comprises double bonds.
 4. The method ofclaim 1, wherein at least a portion of the semiconductor-nanocrystalshave an aspect ratio greater than about
 10. 5. The method of claim 1,wherein at least a portion of the semiconductor-nanocrystals have anaspect ratio of between about 5 and about
 10. 6. The method of claim 1,wherein the semiconductor-nanocrystals comprise tetrapods.
 7. The methodof claim 1, wherein the semiconductor nanocrystals comprise CdSe orCdTe.
 8. The method of claim 1, further comprising disposing thenanocrystals and binder between a pair of opposed electrodes.